Generally, the job of the press section in paper-making is to put roll-pressure on the web which has been dewatered to 15-18% in fiber dryness in the upper stream process, the wire section and to raise its fiber dryness to 40-45% before transfer to the next process, the dryer section.
FIG. 9 depicts one example of a high speed conventional press device used in the press section. In this example, web 2, having a moisture content of about 80% emerges from wire section 1. The web is then stuck to pickup felt 4 and sucked upon by suction pickup roll 3, then dewatered upside and downside in the first press 1P made up of the felt 4, 1P felt 5, suction roll 6, and first grooved roll 7a by porous felts 4 and 5, as indicated by arrows A and B. The web 2 is then held on the pick up felt 4 while it is sucked up by the vacuum of suction roll 6 and dewatered at the second press 2P of single felt comprising a hard roll whose surface is smooth, namely, press center roll 8a, which is granite roll or artificial stone cover roll or rubber cover roll and second grooved roll 7b leaving moisture absorbed by pickup felt 4. Here web 2 is transferred from pickup felt 4 to a more elaborate press center roll 8a and its moisture is reduced to 55-60% by the third press 3P comprising roll 8a and third grooved roll 7c. It is moved to the next press (the fourth press) or the next process, drying.
The operations of passing web in the above-described system is outlined as follows: Web 2, dehydrated by third press 3P, is stuck onto the surface of center roll 8a, doffed by doctor 10 and is temporarily dropped into full-width press pit 11, as shown by 2'. Because this state makes it difficult to feed a full-width web into the next stage press or the dryer section, the method adopted here is to feed a narrow web (tail) first and then widen it to the required width. The first operation is to produce a tail of any width on the wire section using a high-pressure water jet (not illustrated). After either or both of paper roll 15 and lead-in felt roll 17 is moved to the position indicated by a dotted line, as shown by 15' and 17' on FIG. 8, to secure sufficient space for the tail to pass easily through, air is blown off the air nozzle of carrier jet 19, doffing tail from press center roll 8a, putting it on felt 21 for transport to the next-stage press, as shown by 2".
FIG. 10 describes the operations with web 2 being moved from the conventional press center roll 8a to the next fourth press felt 21.
In FIG. 10, the web 2, dehydrated by 3P felt 9, is pressed hard against the side of press center roll 8a due to dehydration or, to be more precise, is stuck onto it. The web 2, stuck on the press center roll 8a, needs to be doffed before it is transferred to the following stage press. Consequently the following stage press is operated faster than the previous process. This additional speed is called "draw".
The draw generates tension in the web and the consequent force pulls the web off the press center roll, as opposed to adhesion. A tensile force commensurate with the draw acts on the full width of the open draw part (portion of the web not supported by felt or roll). This tension stretches the web. The part of the web that is extended is primarily on the area which peels off, the place where pull grows, but dilation varies from part to part because of the uneven fabric quantity, dispersion and additive distribution, producing an uneven strain. If the web is partially destroyed, it is broken off full breadth. The extent of the partially focused elongation increases in proportion to the length of the open draw, raising the risk of paper breakage. It is therefore necessary to shorten the open draw section as much as possible.
An example with such steps is illustrated in FIG. 11. Press center roll 8a and suction roll are laid out to make the open draw part as short as possible and limit it to only one place. In FIG. 11, when the suction roll is pressed against press center roll 8a, aiming it at the open draw to prevent paper breakdown, one will discover that felt and canvas are wound around the suction roll at the same speed as the press center roll. However, no draw will result, the pull of the web being insufficient to peel itself from the center roll. The vacuum pressure of the suction roll doffs the paper only after nipping because of insufficient ventilation of the press center. Consequently, it is impossible to peel the web from the press center roll only with the vacuum pressure of the suction roll, leaving the web stuck on the press center roll due to its adhesion. To tear the web from the press center roll, there must therefore be a draw in place and there is no option but to retain the minimum open draw. After all, a fundamental solution remains to be found to the earlier question of paper breakage.
Paper making machines have become impressively faster in recent years; some conventional newsprint machines operate at a speed of about 1,300 meters per minute. There is a growing need for operations which run at 1,500 meters per minute. One stumbling block to the faster operations of paper making machines is the question of transferring the web from one press to another and from the press section to the dryer.
As explained, the method previously used was to peel the tail of the web with the air nozzle as described above before transfer to the next process. In faster operations however, the web becomes more difficult to doff from the press center roll. To remove the web from the press center in a stable manner, it is necessary to produce a larger draw between presses or the press and the dryer. The web after the press part is 40-45% consistency and its strength is very low compared with dry paper, and is not supported with felt or any others while it is in the above sections. Because of the so-called open draw, paper is apt to break if the draw gets larger. Even if paper is passed through the whole system and normal operations are started, the paper is easily broken by fluctuations.
It is therefore an object of the present invention to provide a web transfer device which better transfers the rolls from one press to another.
Another object of the invention is to provide a web from the press section to the draw.
Still another aspect of the invention is to provide a web transfer device which the paper is not easily broken.